Conventional gardening focuses on plant health; organic gardening focuses on soil health. That’s what the instructor said in the gardening class I was in at Lewis Ginter Botanical Garden. Then she taught us how to do a soil test at home – no mailing a sample off and waiting for results.
I’m working on getting a certification in Gardening and Garden Design, and it’s the most fun I’ve had learning stuff in classes since I studied for my pilot’s license. I look forward to each one and even though I’ve been gardening my whole life I always learn something new.
In the soil class we learned about life underground, and it’s pretty exciting down there. Friendly microbes, insects and worms are all partying down below, creating a happy habitat for plant roots. We learned why healthy soil is the key to organic urban gardening.
Garden soil (never “dirt”) is made up of minerals and organic matter, with lots of gaps for air and water to move through. In fact, our instructor told us good soil texture is half matter, half air and water. Compacting the soil removes the critical air and water pockets that insects and plant roots need.
In the urban garden, soil is often far too compacted from frequent traffic and construction. That’s why it’s a good idea to build raised beds or walkways to keep garden soil light and fluffy.
But not too fluffy. Fluffy, sifted soil will compact quickly. Chunks of soil, called “peds,” rest against each other to create air pockets that remain even after hard rains. The first, and most basic, soil test to do at home is simply check your garden soil for compaction. If it’s compacted, break it up into chunks to add air pockets, and mix in some compost or bagged garden soil to add organic material. Also consider growing a cover crop on any bare ground (and over winter), because the roots will break up compacted soil.
We took a test in the soil class, but not the nail-biter botany type of thing that English majors like me avoid. The instructor showed us how to do a soil test at home – well, two actually: A simple soil test in the field, and a more involved test to do at home. I already knew about testing soil for pH levels (the amount of acid or base in the soil to see how different plants will fare). But these tests identified soil content and texture.
For the field soil test, we got our hands dirty. Grab a fistful of soil and wet it with water until damp. Knead it, adding more water if necessary, until it starts to hang together in a ball. Start pushing the soil out between your thumb and finger to make a ribbon. If the ribbon extends more than an inch without breaking, it has a fair amount of clay.
If it breaks, then rub it between your fingers with more water. Sandy soil feels gritty; silty soil feels smooth.
The second soil test was a little more lab-like. Take a handful of soil and dump it in a pint canning jar. Fill the jar 3/4 full with water, put on the lid, and shake it hard for two minutes. Then, let it sit and separate into layers (this can take up to 24 hours).
The first soil layer to settle is sand, then silt, then clay, then water and floating organic matter. The thickness of the layers corresponds to the percentage of the soil material.
Urban garden soil is often rocky, especially if you dig below grade, where construction crews often dump bricks and other building waste materials. In a rural or suburban garden, the occasional rock is no problem. It will break down eventually, contributing minerals to the soil.
But in the urban garden, we are constantly picking out gravel, glass, brick and rocks. A rocky base will drain well, but make sure you have a good foot or more of healthy soil above that.
Clay isn’t the enemy! This was a shock to me, used to adding compost and yet more compost to our thick Virginia clay. In fact, clay is full of plant food and it hangs onto moisture. This means you can water less frequently.
Water runs right out of sandy soil, and so do plant nutrients. Mix in clay and organic matter to boost it.
Silty soil is a gardener’s friend. It holds moisture well and contains plant nutrients, so it is pretty fertile. But fine silt compacts easily, so a silty soil needs organic matter and some sand to maintain good aeration.
Once I learned how to do a soil test at home, I went back to test all my urban garden beds. I learned that our yard’s soil at grade is heavy on silt and clay – healthier than I expected for an urban space! Our raised beds consist of more sand and compost. I may add a little clay soil from the yard to those.
Ultimately, you can’t have too much organic material. The final word from the instructor for organic urban garden soil is: When in doubt, mix in more compost!